Oil sand, such as is mined in the Fort McMurray region of Alberta, generally comprises water-wet sand grains held together by a matrix of viscous bitumen. It lends itself to liberation of the sand grains from the bitumen by mixing or slurrying the oil sand in water, allowing the bitumen to move to the aqueous phase.
For many years, the bitumen in the McMurray sand has been commercially removed from oil sand using what is commonly referred to in the industry as the “hot water process”, whereby as-mined oil sand is mixed in a rotating tumbler for a prescribed retention time (generally in the range of 2 to 4 minutes) with hot water (approximately 80-90° C.), steam, caustic (e.g., sodium hydroxide) and naturally entrained air to yield a slurry that has a temperature typically around 80° C. The bitumen matrix is heated and becomes less viscous. Chunks of oil sand are ablated or disintegrated. The released sand grains and separated bitumen flecks are dispersed in the water. To some extent bitumen flecks coalesce and grow in size. They may contact air bubbles and coat them to become aerated bitumen. The term used to describe this overall process in the tumbler is “conditioning”. The slurry is then diluted with additional hot water to produce a diluted slurry having a temperature of about 65° C. to about 80° C. The diluted slurry is introduced into a large, open-topped, conical-bottomed, cylindrical vessel commonly termed a primary separation vessel (PSV) where the more buoyant aerated bitumen rises to the surface and forms a bitumen froth layer. This froth layer overflows the top lip of the PSV and is received in a launder extending around the PSV's rim. The product is commonly called “primary bitumen froth” and typically has a temperature of about 65° C. to about 75° C.
In the early 1990s, there was a major innovation in the oil sand industry, which is commonly referred to as “pipeline conditioning”. This innovation is disclosed in Canadian Patent No. 2,029,795 and U.S. Pat. No. 5,264,118. As-mined oil sand is mixed at the mine site (for example, in a cyclofeeder) with hot water, air and NaOH to produce a slurry. The slurry is pumped through a pipeline at least about 2.5 kilometres in length and is fed directly to a conventional gravity separation vessel such as a PSV. In the course of being pumped through the pipeline, sufficient coalescence and aeration of bitumen occurs so that, when subsequently retained in the vessel under quiescent conditions, a desirable amount of the bitumen floats, forms froth, and is recovered.
In the late 1990s a cold dense slurrying process for extracting bitumen from oil sand was developed, which is disclosed in Canadian patent No. 2,217,623 and U.S. Pat. No. 6,007,708. This process is commonly referred to as the “low energy extraction process” or the “LEE process” and generally comprises mixing as-mined oil sand with water in predetermined proportions near the mine site to produce a slurry containing entrained air and having a controlled density in the range of 1.4 to 1.65 g/cc and preferably a temperature in the range 20-40° C.; pumping the slurry through a pipeline having a plurality of pumps spaced along its length, preferably adding air to the slurry as it moves through the pipeline, to condition the slurry; diluting the slurry with flood water; and introducing the diluted slurry into a conventional gravity separation vessel such as a PSV to float the aerated bitumen. The froth is maintained at a temperature of at least 35° C. in the PSV by use of a warm water underwash, thereby assisting in removing the froth from the PSV and satisfying downstream froth temperature needs. A middlings layer and tailings layer are also formed in the PSV. A stream of middlings may be continuously withdrawn and further bitumen recovered in a secondary recovery circuit, for example, mechanical flotation cells. The secondary bitumen froth so produced may either be combined with the primary bitumen froth or recycled and added to the fresh slurry being introduced to the primary separation to increase bitumen recovery as primary froth, as described in U.S. Pat. No. 4,776,949.